It is interesting to see that some of these aluminum materials need to be brazed at temperatures that are only about 10-50°F (5-30°C) below the melting points of those base metals! The reason for this is that, unfortunately, the aluminum-based BFMs available for this kind of work can only melt at temperatures very close to that of the base metals. (Note: Research work continues in an effort to develop effective BFMs that operate at lower temperatures.)
Since aluminum brazing in a vacuum furnace can take place only a few degrees below the melting point of the base metals, furnaces that would be used for this kind of work must be capable of very tight temperature controls – much tighter than can be achieved in standard high-temperature, aerospace-type vacuum furnaces.
To achieve the very close temperature control needed, aluminum-brazing vacuum furnaces typically have many more controllable heating zones than a normal aerospace vacuum furnace. Not only might there be more heating elements around the wall of the furnace (each one separately controlled), but additional heating elements are typically added both on the front door of the furnace and on the back wall (the back wall can often be opened as well). A photo of a typical horizontal aluminum vacuum furnace is shown in Figure 3. With all of these closely controlled heating zones available, an aluminum vacuum-brazing furnace can typically control temperatures to about +/-1° in an empty condition and to about +/-5 to 10°F (about +/-2 to 5°C) when loaded with parts.